Why size matters! (And Why Your Coffee Suddenly Tasted Sour)

If you’ve ever switched brew methods and thought,
“Did my coffee go bad overnight?”
—it didn’t. 
It's likely your grind size betrayed you. We recently got a DM that said:

“I’ve always used your coffee in my Keurig, but when we switched to a French press the coffee tasted sour.”

This is one of the most common coffee problems people run into—and it has nothing to do with the beans. It has everything to do with grind size.

Coffee Extraction (The Non-Boring Explanation)
When you brew coffee, you’re extracting flavor compounds from the ground beans using water. That extraction happens based on three things:

• Time

• Water

• Surface area (this is where grind size comes in)

Smaller grind = more surface area
Larger grind = less surface area

If your grind size doesn’t match your brew method, the coffee will taste wrong—even if the coffee itself is great.

Why It Worked in a Keurig but Failed in a French Press

Keurig / Pod-style brewers brew have a very fast brew time, pressurized water and are designed for a finer grind

French Press has a longer brew time (4–5 minutes), full immersion of the ground, and need a coarse grind

If you take a grind meant for a Keurig and use it in a French press, here’s what happens:

• The water extracts too fast

• It pulls out acids first

• It never gets to the sugars or deeper flavors

That’s called under-extraction. Under-extracted coffee tastes sour, thin, and sharp. Almost lemony (in a bad way). That sour taste isn’t “strong.”
It’s unfinished.

Sour vs Bitter (Quick Myth Busting)
People often mix these up.
Sour coffee = under-extracted
(Grind too coarse, or brew too fast)
Bitter coffee = over-extracted
(Grind too fine, or brew too long)

Most home coffee problems come from grind size—not roast level, not caffeine, not “bad beans.”

The Fix (It’s Easy, We Promise)

If you’re brewing with a French press, you want to use a coarse, sea salt-like grind. If you’re buying pre-ground coffee, make sure it’s ground specifically for the French press, or ask your roaster to grind it for that method. To be clear, the pre-ground coffee you get on the website is more of a Medium grind. Soon, at the shop, we'll have the ability to grind your whole beans to whatever size you prefer. 
 *life hack; If you have whole bean coffee but don't own a good grinder; Take your coffeeto Trader Joe's and use theirs! 😹

Final Thought

Coffee isn’t complicated—but it is specific. If you change the brew method and don’t change the grind, you’re basically switching vehicles without changing the fuel and wondering why it runs like garbage.

The beans weren’t the problem.
The grind was.

And now you know.